Pick any 2-3 from the below:
* Vientiane - just chill, visit bombie museum and temples.
* Luang Prabang - fantastic place to chill, sightseeing. Love this place.
* Plain of Jars (Phonsavan) - Take a day tour in the morning to see the mysterious Plain of Jars. There are several sites, but the tour should cover the main ones. Or if you can ride a motorbike, rent one and go visit on your own (recommend this only if you have a motorbike licence as riding in Laos is dangerous for newbies).
* Van Vieng - used to be a really crazy party town of drugs and alcohol and riverside parties, but authorities have clamped down after tourists died. Not sure it's current status. Last time I was there, it was quite subdued, with lots of restaurants and day tours to the surrounding rivers and caves.
Transport between cities is mostly by minivan or big buses. Minivans are faster, but more dangerous as the drivers are crazy. Prepare to spend half to a full day traveling, so allocate your time accordingly.